The present invention relates to an omnidirectional transducer for elastic waves with a wide pass band. The term elastic waves is understood to mean a pressure wave transmitted in a liquid or a gas. In the latter case an elastic wave is often called a sound or acoustical wave. The transducer can be used in transmission and it then converts an electrical signal into an elastic wave, or in reception and then converts an elastic wave into an electrical signal. The transducer according to the invention is particularly adapted to the transmission of sound and ultrasonic waves.
The submarine transmission of high frequency elastic signals (ultrasonic waves) constitutes a preferred field of application of the present invention. The transducer can be used there like a hydrophone, both as a receiver and a transmitter. The wide pass band of this transducer enables the transmission of signals with a high frequency, such as e.g. television signals specially adapted for transmission in water (pass band of approximately 200 kHz), speech signals or any other information signal.
High fidelity sound reproduction constitutes another preferred field of application for the invention. The transducer used in transmission provides a high performance loudspeaker with a wide pass band.
The principle of the transducer according to the invention is based on the magnetostriction effect. Magnetostriction is the property of certain materials to undergo a geometrical modification (contraction, expansion, bending, twisting, etc.), when subject to the influence of a magnetic field. Metal alloys and in particular ferromagnetic compounds are magnetostrictive materials.
The use of such materials for converting an electric wave into an elastic wave is not new. French Pat. No. 7 702 333 entitled "Electroacoustic transducer with a magnetostrictive core" describes such a device.
This known transducer fundamentally has a magnetostrictive bar arranged in a solenoid. When a voltage signal is applied to the terminals of the coil, it produces in the solenoid axis a magnetic field, which expands or contracts the magnetostrictive bar as a function of the nature of the magnetostrictive material used. At each of the ends of the magnetostrictive bar, this produces an elastic wave which is propagated in a direction substantially parallel to the magnetostrictive bar axis.
Thus, it acts here as a monodirectional electroacoustic transducer, because the elastic wave is only transmitted in the direction of the bar axis. This axial transmission character is not desired in all applications and can constitute a serious disadvantage, particularly in the high fidelity reproduction of sound waves, where it is desired that the elastic wave is transmitted with the same intensity in all directions.
French Pat. No. 8,106,150 entitled "Omnidirectional loudspeaker for very high frequencies of the sound spectrum" describes an electroactoustic transducer usable in air, which makes it possible to produce a substantially omnidirectional elastic wave, on the basis of the linear expansion or linear contraction, as a function of the material used, of a magnetostrictive bar. FIG. 1 shows a loudspeaker construction in accordance with the teaching of this patent.
This loudspeaker essentially comprises two rigid hemispheres 2, 4, which are interconnected by an elastic annular joint 6, to which they can be fixed by bonding, so as to form a pulsating sphere, together with a control element 8 placed within the sphere and rigidly connected to the hemispheres 2, 4.
The control element 8 is elongated and can undergo a length variation in response to an electrical signal which is to be converted into an elastic wave. The control element is oriented within the sphere in such a way that the forces resulting from these length variations are transmitted to the hemispheres 2, 4, in directions perpendicular to the connection plane of said hemispheres.
The length of this control element 8 is preferably less than the diameter of the sphere, and it is then connected to the hemispheres 2, 4 by rigid transmission parts 10, 12, which then engage perpendicularly on hemispheres 2, 4 at locations sufficiently remote from the region of their tops to ensure that each of the said hemispheres is displaced or vibrates continuously, in response to length variations of the control element 8. These transmission parts are substantially shaped like a spherical cap.
The control element 8 is constituted by a bar 14, having a circular or square cross-section and made from a magnetostrictive material, about which is arranged induction coil 16. The electrical signal to be converted into an elastic wave is applied to the ends of the coil 16 by two electrical conductors 18, 20, which pass through an opening formed in annular joint 6.
The transmission parts 10, 12 make it possible to convert the linear expansion of the magnetostrictive bar 14 into a displacement of each of the two hemispheres 2, 4. The elastic wave produced is thus substantially omnidirectional.
However, the wave produced cannot be perfectly omnidirectional, which in part is due to the presence of the elastic annular joint 6 joining the two hemispheres and which consequently prevents the transmission of the sound wave in its plane, whilst also being due to the fact that the transmission parts do not modify the linear character of the transmitted force. Thus, it is not precisely a pulsating sphere, i.e. having a radius which is variable, but identical at each point at a given instant, but is instead merely two hemispheres simultaneously having a linear movement in the same direction and of the opposite sense.